Virtual ground sensing circuitry and related devices, systems, and methods for crosspoint ferroelectric memory

ABSTRACT

Virtual ground sensing circuits, electrical systems, computing devices, and related methods are disclosed. A virtual ground sensing circuit includes a sense circuit configured to compare a reference voltage potential to a sense node voltage potential, and virtual ground circuitry operably coupled to the sense circuit. The virtual ground circuitry is configured to provide a virtual ground at a first bias voltage potential to a conductive line operably coupled to a selected ferroelectric memory cell, and discharge the conductive line to the sense node responsive to the selected ferroelectric memory cell changing from a first polarization state to a second polarization state. A method includes applying a second bias voltage potential to another conductive line operably coupled to the selected ferroelectric memory cell, and comparing a sense node voltage potential to a reference voltage potential. Electrical systems and computing devices include virtual ground sensing circuits.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.14/717,471, filed May 20, 2015, pending, the disclosure of which ishereby incorporated herein in its entirety by this reference.

FIELD

The present disclosure relates generally to detecting charges stored inmemory cells. More specifically, the present disclosure relates todetecting charges stored in ferroelectric memory cells, and to relatedcircuits, devices, systems and methods.

BACKGROUND

Manufacturers of data storage devices continually seek to provide datastorage devices with increased speed (e.g., faster read/writeoperations), lower power consumption, and higher memory capacity.Although many different kinds of data storage devices have beencontemplated to date, NAND Flash memory, which typically includes arraysof floating gate transistors that store different charge levelscorresponding to different digital bit states, remain prominent.

NAND Flash memory remains prominent despite other forms of data storageoffering better speed, and lower power consumption (e.g., ferroelectricmemory). This continued prominence of NAND Flash may be due in part toits relatively low cost of manufacturing, and relatively high storagedensity (i.e., relatively small memory cells), as compared to otherforms of data storage.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified diagram of a portion of an array of ferroelectricmemory cells according to embodiments of the present disclosure;

FIGS. 2A and 2B illustrate relationships between different voltagepotentials applied to a ferroelectric memory cell of the array of FIG. 1and corresponding polarization states of the ferroelectric memory cell106;

FIG. 2A illustrates a ferroelectric memory cell of the array of FIG. 1operably coupled to a hypothetical variable voltage source;

FIG. 2B is a simplified plot showing the relationships of differentvoltages applied to the ferroelectric memory cell to differentpolarization states of the ferroelectric memory cell;

FIG. 3 is a simplified schematic view of a portion of the array of FIG.1, according to embodiments of the disclosure;

FIG. 4 is a simplified block diagram of control circuitry operablycoupled to one of the ferroelectric memory cells of the array of FIG. 1;

FIG. 5A is a schematic diagram of virtual ground sensing circuitry ofthe control circuitry of FIG. 4;

FIG. 5B illustrates plots of voltage potentials from the virtual groundsensing circuit of FIG. 5A during a sense operation of a selectedferroelectric memory cell in a single-level polarization scheme;

FIG. 5C illustrates other plots of voltage potentials from the virtualground sensing circuit of FIG. 5A during a sense operation of a selectedferroelectric memory cell in a single-level polarization scheme;

FIGS. 6A and 6B illustrate a sense circuit and plots of related voltagepotentials according to embodiments of the disclosure;

FIG. 6A is a schematic view of a sense circuit in a multi-levelpolarization scheme;

FIG. 6B illustrates plots of voltage potentials of the sense circuit ofFIG. 6A;

FIGS. 7A and 7B illustrate another sense circuit and plots of relatedvoltage potentials according to embodiments of the disclosure;

FIG. 7A is a schematic view of a sense circuit in a multi-levelpolarization scheme;

FIG. 7B illustrates plots of voltage potentials of the sense circuit ofFIG. 7A;

FIG. 8 is a simplified flowchart illustrating a method of performing asense operation of a selected ferroelectric memory cell; and

FIG. 9 is a simplified block diagram of a computing device including amemory device that includes the control circuitry of FIG. 4.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments in which the present disclosuremay be practiced. These embodiments are described in sufficient detailto enable those of ordinary skill in the art to practice the presentdisclosure. It should be understood, however, that the detaileddescription and the specific examples, while indicating examples ofembodiments of the present disclosure, are given by way of illustrationonly and not by way of limitation. From this disclosure, varioussubstitutions, modifications, additions rearrangements, or combinationsthereof within the scope of the present disclosure may be made and willbecome apparent to those of ordinary skill in the art.

In accordance with common practice, the various features illustrated inthe drawings may not be drawn to scale. The illustrations presentedherein are not meant to be actual views of any particular apparatus(e.g., device, system, etc.) or method, but are merely idealizedrepresentations that are employed to describe various embodiments of thepresent disclosure. Accordingly, the dimensions of the various featuresmay be arbitrarily expanded or reduced for clarity. In addition, some ofthe drawings may be simplified for clarity. Thus, the drawings may notdepict all of the components of a given apparatus or all operations of aparticular method.

Information and signals described herein may be represented using any ofa variety of different technologies and techniques. For example, data,instructions, commands, information, signals, bits, symbols, and chipsthat may be referenced throughout the description may be represented byvoltages, currents, electromagnetic waves, magnetic fields or particles,optical fields or particles, or any combination thereof. Some drawingsmay illustrate signals as a single signal for clarity of presentationand description. It should be understood by a person of ordinary skillin the art that the signal may represent a bus of signals, wherein thebus may have a variety of bit widths and the present disclosure may beimplemented on any number of data signals including a single datasignal.

The various illustrative logical blocks, modules, circuits, andalgorithm acts described in connection with embodiments disclosed hereinmay be implemented as electronic hardware, computer software, orcombinations of both. To clearly illustrate this interchangeability ofhardware and software, various illustrative components, blocks, modules,circuits, and acts are described generally in terms of theirfunctionality. Whether such functionality is implemented as hardware orsoftware depends upon the particular application and design constraintsimposed on the overall system. Skilled artisans may implement thedescribed functionality in varying ways for each particular application,but such implementation decisions should not be interpreted as causing adeparture from the scope of the embodiments of the disclosure describedherein.

In addition, it is noted that the embodiments may be described in termsof a process that is depicted as a flowchart, a flow diagram, astructure diagram, or a block diagram. Although a flowchart may describeoperational acts as a sequential process, many of these acts can beperformed in another sequence, in parallel, or substantiallyconcurrently. In addition, the order of the acts may be rearranged. Aprocess may correspond to a method, a function, a procedure, asubroutine, a subprogram, etc. Furthermore, the methods disclosed hereinmay be implemented in hardware, software, or both. If implemented insoftware, the functions may be stored or transmitted as one or morecomputer-readable instructions (e.g., software code) on acomputer-readable medium. Computer-readable media may include bothcomputer storage media and communication media including any medium thatfacilitates transfer of a computer program from one place to another.Computer-readable media may include volatile and non-volatile memory,such as, for example, magnetic and optical storage devices, such as, forexample, hard drives, disk drives, magnetic tapes, CDs (compact discs),DVDs (digital versatile discs or digital video discs), solid statestorage devices (solid state drives), and other similar storage devices.

It should be understood that any reference to an element herein using adesignation such as “first,” “second,” and so forth does not limit thequantity or order of those elements, unless such limitation isexplicitly stated. Rather, these designations may be used herein as aconvenient method of distinguishing between two or more elements orinstances of an element. Thus, a reference to first and second elementsdoes not mean that only two elements may be employed there or that thefirst element must precede the second element in some manner. Also,unless stated otherwise a set of elements may comprise one or moreelements.

Elements described herein may include multiple instances of the sameelement. These elements may be generically indicated by a numericaldesignator (e.g., 106) and specifically indicated by the numericalindicator followed by a numeric indicator preceded by a “dash” (e.g.,106-11). For ease of following the description, for the most part,element number indicators begin with the number of the drawing on whichthe elements are introduced or most fully discussed. Thus, for example,element identifiers on a FIG. 1 will be mostly in the numerical format1xx and elements on a FIG. 3 will be mostly in the numerical format 3xx.

Virtual ground sensing circuits disclosed herein may be configured toperform sense operations by discharging a conductive line operablycoupled to a selected memory cell to a sense node having a sense nodecapacitance less than a capacitance of the conductive line. Although thepresent disclosure is discussed primarily with reference toferroelectric memory, the systems and methods of the present disclosuremay be implemented in any environment where sensing relatively smallcharges and voltage potentials may be helpful or desirable.

As used herein, the term “ferroelectric material” refers to materialsthat demonstrate non-linear polarization in two or more polarizationlevels responsive to different voltage potentials being applied acrossthe materials. By way of non-limiting example, ferroelectric materialsmay include lead zirconate titanate (PZT), strontium bismuth tantalite(SBT), bismuth lanthanum titanate (BLT), lead titanate, barium titanate,and hafnium dioxide (HfO₂). As used herein, the term “polarizationlevels” refers to different magnitudes and orientations (e.g.,directions) of polarization e.g., forward and reverse polarizations inresponse to applied forward and reverse voltage potentials,respectively). In some embodiments, different polarization levels mayalso be subdivided into different energy levels of polarization (i.e.,multi-level charge injection) within the same orientation ofpolarization (enabling multiple bits to be stored to the sameferroelectric memory cell). Multilevel charge injection of ferroelectricmaterials may be used to manufacture multi-bit ferroelectric memorycells. For example, multi-bit ferroelectric memory cells are disclosedin U.S. Pat. No. 6,856,534 to Rodriguez et al., the entire disclosure ofwhich is hereby incorporated herein by this reference.

As used herein, the term “ferroelectric memory cell” refers to a memorycell that includes a ferroelectric material. Different polarizationlevels of the ferroelectric material may correspond to different databits (e.g., a positive polarization may correspond to a logic “1,” and anegative polarization may correspond to a logic “0”, or vice versa). Thepolarization of the ferroelectric material may be controlled and read byapplying specific voltage potentials across the ferromagnetic material,as will be discussed in more detail herein. Although ferroelectricmemory cells including ferroelectric field effect transistors (FeFETs)are known in the art, as used herein, the term “ferroelectric memorycell” refers specifically to a ferroelectric memory cell including aferroelectric capacitor (i.e., a ferroelectric material between twoconductive electrodes).

As used herein, the term “virtual ground” refers to a node of anelectrical circuit that is held at a reference voltage potential, butthat is not operably coupled directly to that reference voltagepotential. Accordingly, a voltage potential of a virtual ground maytemporarily fluctuate (i.e., as transients are applied to the virtualground), but return to the reference voltage potential in the steadystate.

As used herein, the term “charge” refers to a measure of electricalcharge, commonly measured in Coulombs (C). The term “charge” does notherein refer specifically to charge carriers (e.g., electrons, holes).Accordingly, as used herein, terms relating to the phrases “emitting acharge” and “sinking a charge” are used with reference to changes inpositive charge as measured in Coulombs, not specifically to an emissionor sinking of a particular charge carrier.

FIG. 1 is a simplified diagram of a portion of an array 100 offerroelectric memory cells 106 according to embodiments of the presentdisclosure. In the example illustrated in FIG. 1, the array 100 is across-point array having ferroelectric memory cells 106 located at theintersections of a first number of conductive lines 104-1, 104-2, . . ., 104-M (e.g., access lines, which may also be referred to herein asword lines 104), and a second number of conductive lines 102-1, 102-2, .. . , 102-N (e.g., data sense lines, which may also be referred toherein as bit lines 102). Each ferroelectric memory cell 106 may includea ferroelectric capacitor including ferroelectric material coupledbetween two conductive electrodes (e.g., a word line 104 and a bit line102). As illustrated in FIG. 1, the word lines 104 may be substantiallyparallel to each other and substantially orthogonal to the bit lines102, which may be substantially parallel to each other; however,embodiments are not so limited. In the embodiment illustrated in FIG. 1,the ferroelectric memory cells 106 (e.g., 106-11, 106-21, 106-N1, and106-NM) can function in a two-terminal architecture with a particularword line 104 and a particular bit line 102 serving as a top electrodeand bottom electrode, respectively, for the ferroelectric memory cell106. Although in FIG. 1 the array is illustrated in an orientation withbit lines 102 on the bottom, ferroelectric memory cells 106 in themiddle, and word lines 104 on top, other orientations are alsocontemplated. For example, an orientation is contemplated where wordlines 104 are on the bottom, ferroelectric memory cells 106 are in themiddle, and bit lines 102 are on the top. Also, sideways orientationsare contemplated. Furthermore, three-dimensional arrays of ferroelectricmemory cells 106 in a variety of orientations are contemplated (e.g.,multiple tiers of arrays similar to the array 100 of FIG. 1, usingpillar structures such as those used in three-dimensional NAND memory,etc.) Any orientation having ferroelectric memory cells 106 between bitlines 102 and word lines 104 are contemplated within the scope of thedisclosure.

It is noted that the structure of the array 100 is relatively simple,and scaling of the array 100 may be subject essentially to the limits oflithography. Accordingly, it may be possible to manufacture the array100 with relatively small ferroelectric memory cells 106.

It is also contemplated within the scope of the disclosure thatswitching elements (not shown) (e.g., access transistors activated byadditional conductive lines, non-ohmic-devices (NODs) in series with theferroelectric memory cells 106, etc.) may be placed between theferroelectric memory cells 106 and at least one of the bit lines 102 andthe word lines 104. In some embodiments, these switching elements arenot included, as will be discussed in more detail with respect to anexample of a portion 300 of the array 100 with reference to FIG. 3.

In operation, the ferroelectric memory cells 106 of the array 100 can beprogrammed via programming signals (e.g., write voltage potentials)applied to the ferroelectric memory cells 106 (e.g., the ferroelectricmaterial of the cells) via selected word lines 104, bit lines 102, othersuitable signals coupled to the ferroelectric memory cells 106, andcombinations thereof. The amplitude, shape, duration, and/or number ofprogramming pulses, for example, applied to the ferroelectric memorycells 106 can be adjusted (e.g., varied) in order to program theferroelectric material of the ferroelectric memory cells 106 to one of anumber of different polarization levels corresponding to particular datastates.

In some embodiments, an electrical system includes an array of memorycells, each memory cell in the array of memory cells includes a firstelectrode and a second electrode. Each memory cell in the array ofmemory cells is configured to transition from a first data state to asecond data state when a critical voltage potential is applied acrossthe first electrode and the second electrode. The electrical system alsoincludes control circuitry operably coupled to the array of memorycells. The control circuitry includes biasing circuitry configured toapply the critical voltage potential to a selected memory cell of thearray of memory cells by providing a first bias voltage potential to thefirst electrode of the selected memory cell and a second bias voltagepotential to the second electrode of the selected memory cell. Thecontrol circuitry also includes a virtual ground sensing circuitoperably coupled between the biasing circuitry and the second electrode.The virtual ground sensing circuitry is configured to relay the secondbias voltage potential from the biasing circuitry to the secondelectrode, and serve as a virtual ground operably coupled to the secondelectrode. The virtual ground sensing circuitry is also configured todischarge the second electrode to a sense node if the memory celltransitions from the first data state to the second data state. Thevirtual ground sensing circuitry is further configured to compare asense node voltage potential of the sense node to a reference voltagepotential to determine a data state the selected memory cell was in. Insome embodiments, the at least a portion of the memory cells of thearray of memory cells includes a ferroelectric material between thefirst electrode and the second electrode. In some embodiments, theferroelectric material comprises at least one material selected from thegroup consisting of lead zirconate titanate (PZT), strontium bismuthtantalite (SBT), bismuth lanthanum titanate (BLT), lead titanate, bariumtitanate, and hafnium dioxide (HfO₂).

FIGS. 2A and 2B illustrate relationships between different voltagepotentials applied to a ferroelectric memory cell 106 of the array 100of FIG. 1 and corresponding polarization states P1, P2, P3, P4, etc., ofthe ferroelectric memory cell 106. FIG. 2A illustrates the ferroelectricmemory cell 106 of the array 100 of FIG. 1 operably coupled to ahypothetical variable voltage source 210. FIG. 2B is a simplified plot230 showing the relationships of different voltages applied to theferroelectric memory cell 106 to different polarization states P1, P2,P3, P4, etc., of the ferroelectric memory cell 106. As used herein, theterm “polarization state” and “polarization states,” identify differentpolarization levels without making explicit the magnitude (e.g., P_(R1),P_(R2)) and orientation (e.g., +, −) of the corresponding polarizationlevel (e.g., to simplify references to the various polarization levels).For non-limiting examples expressed herein, polarization states P1, P2,P3, and P4 correspond to polarization levels P_(R1), −P_(R1), P_(R2),−P_(R2), respectively.

Referring to FIGS. 2A and 2B together, the ferroelectric memory cell 106may include a ferroelectric material 220 having a first side 222, and asecond side 224 opposite the first side 222. The ferroelectric material220 may have an area A taken substantially parallel to the first andsecond sides 222, 224.

The ferroelectric material 220 may be configurable in a number ofdifferent polarization states P1, P2, P3, P4, etc. Polarization levelscorresponding to the different polarization states P1, P2, P3, P4, etc.,may be differentiated from each other by at least one of orientation(sometimes expressed as positive + and negative −) and magnitude(sometimes expressed in μC/cm²). By way of non-limiting example, apolarization state P1 of the ferroelectric material 220 may correspondto a polarization level having an orientation pointing from the firstside 222 to the second side 224 (hereinafter referred to as a “positiveorientation,” or a “positive polarization”), and a relatively smallmagnitude (e.g., +P_(R1), a residual polarization according to a firstcharge injection level). A polarization state P2 may correspond to apolarization level having an orientation pointing from the secondelectrode 204 to first electrode 202 (hereinafter referred to as a“negative orientation,” or a “negative polarization”), and relativelysmall magnitude (e.g., −P_(R1)). A polarization P3 may correspond to apolarization level having a positive polarization and a relativelylarger magnitude (e.g., +P_(R2), a residual polarization according to asecond charge injection level). A polarization P4 may correspond to apolarization level having a negative polarization, and relatively largermagnitude (e.g., −P_(R2)). In some embodiments, more polarization statesmay exist. By way of non-limiting example, polarization levels havingpositive and negative orientations, and even larger magnitude thanpolarization level P_(R2) are contemplated.

The ferroelectric memory cell 106 may also include a first electrode 202operably coupled to the first side 222 of the ferroelectric material220, and a second electrode 204 operably coupled to the second side 224of the ferroelectric material 220. The first and second electrodes 202,204 may include conductive material. In some embodiments, the bit line102 and the word line 104 of the array 100 (FIG. 1) may include thefirst and second electrodes 202, 204, respectively. In such embodiments,the ferroelectric memory cell 106 may include the ferroelectric material220 between the bit line 102 and the word line 104. In some embodiments,the first electrode 202 and the second electrode 204 may includeconductive structures in addition to the bit line 102 and the word line104, configured to operably couple the ferroelectric material 220 to thebit line 102 and the word line 104, respectively.

As illustrated in FIG. 2A, bias voltages may be applied to the first andsecond electrodes 202, 204. By way of non-limiting example, ahypothetical variable voltage source 210 may be operably coupled to thefirst and second electrodes 202, 204 of the ferroelectric memory cell106. The plot 230 of FIG. 2B illustrates an ascending curve 234 and adescending curve 232 of the polarization P of the ferroelectric material220 as a voltage V of the hypothetical variable voltage source 210 isswept from low to high, and from high to low, respectively, according toa first level of charge injection (i.e., voltage V is swept from a lowvoltage greater than critical voltage −V_(CR2) and less than or equal tocritical voltage −V_(CR1) to a high voltage greater than or equal tocritical voltage V_(CR1) and less than critical voltage V_(CR2)). Theplot 230 also illustrates an ascending curve 238 and a descending curve236 of the polarization P of the ferroelectric material 220 as a voltageV of the hypothetical variable voltage source 210 is swept from low tohigh, and from high to low, respectively, according to a second level ofcharge injection (i.e., voltage V is swept from a low voltage less thanor equal to critical voltage −V_(CR2) to a high voltage greater than orequal to V_(CR2).

Critical voltage V_(CR1) may be a voltage potential that is required toswitch the ferroelectric material 220 from polarization states P2 and P4to polarization state P1. Critical voltage −V_(CR1) may be a voltagepotential that is required to switch the ferroelectric material 220 frompolarization states P1 and P3 to polarization state P2. Critical voltageV_(CR2) may be a voltage potential that is required to switch theferroelectric material 220 from polarization states P1, P2, and P4 topolarization state P3. Similarly, critical voltage −V_(CR2) may be avoltage potential that is required to switch the ferroelectric material220 from polarization states P1, P2, and P3 to polarization state P4. Ingeneral, as used herein, the term “critical voltage,” refers to avoltage potential that, when applied to the ferroelectric memory cell106, causes the ferroelectric material 220 of the ferroelectric memorycell 106 to change from one polarization state to another polarizationstate.

As is apparent from the plot 230, it is possible for the ferroelectricmaterial 220 to be polarized to any of several different polarizationstates P1, P2, P3, and P4 (e.g., corresponding to polarization levels+P_(R1), −P_(R1), +P_(R2), and −P_(R2)) when bias voltages are releasedfrom the ferroelectric material 220 (i.e., zero volts is applied to theferroelectric material 220), depending on previous bias voltagepotentials applied to the ferroelectric material 220. For example,following an application of critical voltage V_(CR2) to theferroelectric material 220, when the critical voltage V_(CR2) isremoved, and the voltage potential across the ferroelectric material 220returns to zero, the polarization state of the ferroelectric material220 may be P3 (corresponding to polarization level +P_(R2)). Thisdependence upon past biases of the ferroelectric material 220 is knownas “hysteresis.”

As is apparent from the plot 230, the ferroelectric material 220exhibits residual polarization, or non-zero polarization level thatremains after voltage potentials applied thereto are removed (i.e.,switched to zero). Polarization states P1, P2, P3, and P4 may correspondto residual polarizations +P_(R1), −R_(R1), +P_(R2), and −P_(R2) of theferroelectric material 220. In general, ferroelectric materials oftenhave a constant known as a “residual polarization” P_(R) associatedtherewith. The residual polarization P_(R) of the ferroelectric material220 may be the magnitude of P1 (i.e., +P_(R1)), in some instances. Theresidual polarization P_(R) (and the different polarization states P1,P2, P3, and P4) may be different for different ferroelectric materials.By way of non-limiting example, the residual polarization P_(R) of PZT,SBT, and BLT may respectively be 25, 10, and 15 micro-Coulombs persquare centimeter (μC/cm²).

In some embodiments, a single level ferroelectric memory cell 106 may beprogrammed to one of two polarization states P1, P2 (e.g., +P_(R1),−P_(R1)), corresponding to a logic 1 and a logic 0 (or vice versa),respectively. By way of non-limiting example, The ferroelectric memorycell 106 may be programmed with critical voltage VT_(CR1), which willplace the ferroelectric memory cell 106 in polarization state P1 (andthe corresponding data state) when the critical voltage V_(CR1) isremoved, or the ferroelectric memory cell 106 may be programmed with acritical voltage −V_(CR1), which will place the ferroelectric memorycell 106 in polarization state P2 (and the corresponding data state)when the critical voltage −V_(CR1) is removed.

In some embodiments, a multi-level ferroelectric memory cell 106 may beprogrammed to one of four or more polarization states P1, P2, P3, P4,etc., corresponding to four or more data states (e.g., digital datastates 00, 01, 10, 11, etc.). The ferroelectric memory cell 106 may beprogrammed with any of several different critical voltages V_(CR1),−V_(CR1), V_(CR2), −V_(CR2), etc., which will place the ferroelectricmemory cell 106 in a corresponding one of the polarization states P1,P2, P3, P4, etc. (and the corresponding data states), when thecorresponding critical voltage V_(CR1), −V_(CR1), V_(CR2), −V_(CR2),etc., is removed. It should be noted that applying critical voltageV_(CR1) to a ferroelectric memory cell 106 in a P3 polarization statemay not switch the ferroelectric memory cell 106 to the P1 state, andapplying critical voltage −V_(CR1) to a ferroelectric memory cell 106 ina P4 polarization state may not switch the ferroelectric memory cell 106to the P2 state.

A different amount of charge may be held by the ferroelectric memorycell 106 depending, at least in part, on the polarization state P1, P2,P3, P4, etc., of the ferroelectric memory cell 106. Accordingly, chargeis emitted from or sinked to the ferroelectric memory cell 106 throughat least one of the first and second electrodes 202, 204 when theferroelectric memory cell 106 switches between the differentpolarization states P1, P2, P3, P4, etc. In the case where polarizationstates P1 and P2 are selected to coincide with +P_(R1) and −P_(R1),respectively, the difference in charge held by the ferroelectric memorycell 106 in polarization state P1 and polarization state P2 is:

ΔQ=2(A)(P _(R1)).

Thus, it follows that when the ferroelectric memory cell 106 switchesfrom polarization state P2 to polarization state P1, the ferroelectricmemory cell 106 sinks positive charge with magnitude 2(A)(P_(R1)). Also,when the ferroelectric memory cell 106 switches from polarization stateP1 to polarization state P2, the ferroelectric memory cell 106 emitspositive charge with magnitude 2(A)(P_(R1)). Similarly, chargeproportional to the differences between polarization states P1, P2, P3,P4, etc., may be emitted or sinked when switching between thepolarization states P1, P2, P3, P4, etc. (e.g., charge is sinked whenswitching from P1 to P3, from P4 to P1, from P4 to P3, and from P2 toP3; and charge is emitted when switching from P2 to P4, from P3 to P2,from P3 to P4, and from P1 to P3). Therefore, it is possible todetermine a polarization state P of the ferroelectric memory cell 106 ifa known critical voltage V_(CR1), −V_(CR1), V_(CR2), −V_(CR2), etc., isapplied to the ferroelectric memory cell 106, and charge emitted from orsinked to the ferroelectric memory cell 106 (or lack of charge emittingfrom or sinking to the ferroelectric memory cell 106) is measured. Itshould be noted that since switching from P4 to P2, and from P3 to P1may not be readily performed by applying the respective criticalvoltages V_(CR2) and V_(CR1), it may be helpful to apply either V_(CR2)or −V_(CR2) to the ferroelectric memory cell 106 to determine which ofthe polarization states P1, P2, P3, P4 the ferroelectric memory cell 106is in if a multi-level scheme is used.

A sense operation (e.g., a read operation), therefore, may be used todetermine the polarization state P (and by extension, the correspondingdata state) of the ferroelectric memory cell 106. The sensing operationmay include applying a critical voltage V_(CR1), −V_(CR1), V_(CR2),−V_(CR2), etc., to the ferroelectric memory cell 106 (destroying thedata stored on the ferroelectric memory cell 106). The sense operationmay also include sensing charge emitted to/sinked from the ferroelectricmemory cell 106 (e.g., to one of the bit line 102 and the word line 104)while applying the critical voltage V_(CR1), −V_(CR1), V_(CR2), −V_(CR2)to the ferroelectric memory cell 106. If the information stored in theferroelectric memory cell 106 is desired to persist past the senseoperation, the critical voltage corresponding to the sensed polarizationstate may be re-applied to the sensed ferroelectric memory cell 106(unless the critical voltage used to perform the sense operationcorresponds to the sensed polarization state, in which case the sensedferroelectric memory cell 106 will already be in the appropriatepolarization state).

The magnitude of charge emitted from or sinked to the ferroelectricmemory cell 106 when the ferroelectric memory cell 106 switches from onepolarization state to another polarization state is proportional to thearea A of the ferroelectric memory cell 106. A market demand forcontinually higher and higher density memory cell arrays (andcorrespondingly smaller and smaller memory cell areas) suggests that allother things being equal, the area A of the ferroelectric memory cell106 should preferably be as small as possible. Since the magnitude ofcharge emitted from or sinked to the ferroelectric memory cell 106 isproportional to the area A, which is preferably selected to berelatively small, the magnitude of the charge emitted or sinked maylikewise be relatively small.

A change in voltage on one of the bit line 102 and the word line 104responsive to the polarization switch is proportional to the chargeemitted from or sinked to the ferroelectric memory cell 106 (i.e.,ΔV=ΔQ/C, where ΔV is the change in voltage, ΔQ is the change in charge,and C is the capacitance of the conductive line (e.g., the bit line 102or the word line 104) that receives/provides the charge). Assuming thatthe capacitance of the conductive line 102, 104 (C) is about a picoFarad(pF), the residual polarization P_(R) is about 25 μC/cm², and the area Aof the ferroelectric memory cell 106 is about 100 nm², switching betweenpolarization states P1 and P2 would result in a change in voltage ΔV ofonly about 50 microvolts (μV) on the conductive line 102, 104. Virtualground sensing circuitry 500 that may be capable of detecting therelatively small change in voltage will be discussed below withreference to FIGS. 4 through 7B.

FIG. 3 is a simplified schematic view of a portion 300 of the array 100of FIG. 1, according to some embodiments of the disclosure. In theseembodiments, it is assumed that a single-level polarization scheme isused (e.g., each memory cell 106 stores a single bit with a logic level1 assigned to a polarization state P1 (i.e., +P_(R1)), and a logic level0 is assigned to a polarization state P2 (i.e., −P_(R1)), or viceversa). Access switches (e.g., access transistors) may be needed inmulti-level polarization schemes. The portion 300 of the array 100 mayinclude ferroelectric memory cells 106-11, 106-21, 106-12, 106-22(sometimes referred to generally individually as “ferroelectric memorycell” 106 and together as “ferroelectric memory cells” 106) operablycoupled between bit lines 102-1, 102-2 (sometimes referred to generallyindividually as “bit line” 102 and together as “bit lines” 102) and wordlines 104-1, 104-2 (sometimes referred to generally individually as“word line” 104 and together as “word lines” 104). Bias voltages (e.g.,critical voltages) may be applied to the bit lines 102 and the wordlines 104 to access (e.g., sense, set, reset) the ferroelectric memorycells 106. One or more of the ferroelectric memory cells 106 may beselected for accessing at a time.

By way of non-limiting example, ferroelectric memory cell 106-21 may beselected and reset to the polarization state P2 (FIGS. 2A and 2B). Afirst bias voltage potential (VG+V_(CR1)/2) equal to a virtual groundpotential (VG) minus half of critical voltage −V_(CR1) (FIGS. 2A and 2B)may be applied to bit line 102-1. A second bias voltage potential(VG−V_(CR1)/2) equal to the virtual ground potential (VG) plus half ofthe critical voltage −V_(CR1) may be applied to word line 104-2. As aresult, a total of −V_(CR1) (the critical voltage −V_(CR1)) may beapplied across the ferroelectric memory cell 106-21. The virtual groundvoltage VG may be applied to bit line 102-2 and word line 104-1.Accordingly, although voltage potentials having magnitudes equal to halfthe critical voltage −V_(CR1) are applied to unselected memory cells106-11 and 106-22, half the critical voltage −V_(CR1) is not sufficientto change the polarization state of the unselected memory cells 106-11and 106-22. A voltage applied across unselected memory cell 106-12 maybe about zero volts, which is also insufficient to switch the memorycell 106-12 away from either the P1 or P2 polarization state. Thus,access transistors may not be needed in this embodiment.

Also by way of non-limiting example, ferroelectric memory cell 106-21may be selected and set to the polarization state P1 (FIGS. 2A and 2B)in a write operation, or in a sense operation. It should be noted thatbecause sense operations in this embodiment place the memory cell 106-21into the polarization state P1, after a sense operation, the memory cell106-21 may need to be reset to the polarization state P2 if it isdetected that the memory cell 106-21 was in the polarization state P2 inorder to preserve the data stored in the memory cell 106-21. A firstbias voltage potential (VG−V_(CR1)/2) equal to the virtual groundpotential VG minus half of critical voltage V_(CR1) (FIGS. 2A and 2B)may be applied to bit line 102-1. A second bias voltage potential(VG+_(VCR1)/2) equal to the virtual ground potential (VG) plus half ofthe critical voltage V_(CR1) may be applied to word line 104-2. As aresult, a total of V_(CR1) (the critical voltage V_(CR1)) may be appliedacross the ferroelectric memory cell 106-21. The virtual ground voltageVG may be applied to bit line 102-2 and word line 104-1. Accordingly,although voltage potentials having magnitudes equal to half the criticalvoltage V_(CR1) are applied to unselected memory cells 106-11 and106-22, half the critical voltage V_(CR1) is not sufficient to changethe polarization state of the unselected memory cells 106-11 and 106-22.Also, a voltage applied across unselected memory cell 106-12 may beabout zero volts, which is also insufficient to switch the memory cell106-12 away from either the P1 or P2 polarization state.

While the examples discussed above with reference to FIG. 3 refer toselecting and accessing ferroelectric memory cell 106-21, the otherferroelectric memory cells 106-11, 106-12, 106-22 may similarly beselected and accessed. Also, all the ferroelectric memory cells 106 maybe selected and accessed simultaneously (e.g., during a reset operationof all the ferroelectric memory cells 106). For example, a first biasvoltage (VG−V_(CR)/2) equal to the virtual ground voltage VG minus halfof one of the critical voltages V_(CR1), −V_(CR1) may be applied to bothbit lines 102-1, 102-2, and a second bias voltage (VG+V_(CR)/2) equal tothe virtual ground voltage VG plus half of the one of the criticalvoltages V_(CR1), −V_(CR1) may be applied to both word lines 104-1,104-2. As a result the one of the critical voltages V_(CR1), −V_(CR1)may be applied to each of the ferroelectric memory cells 106. It shouldbe apparent to those of ordinary skill in the art how to similarlyselect and access individual ferroelectric memory cells 106, subgroupsof ferroelectric memory cells 106, and all ferroelectric memory cells ina full array 100 using similar techniques.

FIG. 4 is a simplified block diagram of control circuitry 400 operablycoupled to one of the ferroelectric memory cells 106 of the array 100 ofFIG. 1. The control circuitry 400 may be configured to access theferroelectric memory cell 106. The control circuitry 400 may includebiasing circuitry 410, virtual ground sensing circuitry 500, and a bitline decoder 430. In some embodiments, the control circuitry 400 mayinclude a word line decoder (not shown) instead of, or in addition to,the bit line decoder 430.

The biasing circuitry 410 may be configured to provide bias voltagepotentials for biasing the bit lines 102 and the word lines 104 of thearray 100 (FIG. 1). For example, the biasing circuitry 410 may beconfigured to provide a word line bias voltage potential W/L BIAS and abit line bias voltage potential B/L BIAS. W/L BIAS and B/L BIAS may beselected to enable the virtual ground sensing circuitry 500 to accessthe ferroelectric memory cell 106 (e.g., perform sense, set, and resetoperations as previously discussed). By way of non-limiting example, W/LBIAS and B/L BIAS may be selected to apply the various critical voltagesV_(CR1), −V_(CR1), V_(CR2), −V_(CR2) to the ferroelectric memory cell106.

The biasing circuitry 410 may also be configured to provide referencevoltage potentials V_(REF) to the virtual ground sensing circuitry 500.The virtual ground sensing circuitry 500 may use the reference voltagepotentials V_(REF) in sense operations to detect charge sinked to oremitted from the ferroelectric memory cell 106. By way of non-limitingexample, the virtual ground sensing circuitry 500 may compare thereference voltage potentials V_(REF) to changes in voltage potentialsthat occur when the ferroelectric memory cell 106 sinks and/or emitscharge during sense operations. The virtual ground sensing circuitry 500may output data signals DATA (latch 440) indicating a data statecorresponding to a sensed polarization state of the ferroelectric memorycell 106.

The virtual ground sensing circuitry 500 may also be configured toprovide a virtual ground at a voltage potential B/L BIAS′ equal to thebit line bias voltage potential B/L BIAS to the bit line 102 (e.g.,through the bit line decoder 430). In other words, the virtual groundsensing circuitry 500 may be configured to relay the bit line biasvoltage potential B/L BIAS to the bit line 102. The virtual groundsensing circuitry 500 may be configured to discharge the bit line 102 toa sense node when charge is sinked to/emitted from the ferroelectricmemory cell 106. The sense node may have a sense node capacitance lessthan a capacitance of the bit line 102. Although FIG. 4 illustrates thevirtual ground sensing circuitry 500 operably coupled to the bit line102 through the bit line decoder 430, in some embodiments the virtualground sensing circuitry 500 may be operably coupled through a word linedecoder to the word line 104 (to sense charge emitted by and sinked tothe ferroelectric memory cell 106 through the word line 104 during senseoperations) instead of, or in addition to the bit line 102.

In some embodiments, a virtual ground sensing circuit includes at leastone sense circuit configured to compare a reference voltage potential toa sense node voltage potential of a sense node of the sense circuit. Thesense node has a sense node capacitance. The virtual ground sensingcircuit also includes virtual ground circuitry operably coupled to theat least one sense circuit. The virtual ground circuitry is configuredto provide a virtual ground at a first bias potential to a conductiveline operably coupled to a selected ferroelectric memory cell. Thevirtual ground circuitry is also configured to discharge the conductiveline to the sense node of the sense circuit responsive to the selectedferroelectric memory cell changing from a first polarization state to asecond polarization state. In some embodiments, the at least one sensecircuit includes a transistor configured to selectively operably couplethe sense node to a power voltage potential. In some embodiments, thetransistor is configured to isolate the sense node from the powervoltage potential during a sense operation of the selected ferroelectricmemory cell. In some embodiments, the at least one sense circuit isconfigured to detect charge emitted by the selected ferroelectric memorycell when the selected ferroelectric memory cell changes from the firstpolarization state to the second polarization state. In someembodiments, the at least one sense circuit is configured to detectcharge sinked to the selected ferroelectric memory cell if the selectedferroelectric memory cell changes from the first polarization state tothe second polarization state. In some embodiments, the sense nodecapacitance is less than a capacitance of the conductive line. In someembodiments, the sense node capacitance includes a parasiticcapacitance.

In some embodiments, the first polarization state and the secondpolarization state correspond to data states in a one-bit single-levelpolarization scheme. In some embodiments, the first polarization statecorresponds to one of a digital logic “1” and a digital logic “0,” andthe second polarization state corresponds to the other of the digitallogic “1” and the digital logic “0.” In some embodiments, the firstpolarization state and the second polarization state correspond to datastates in a multi-bit multi-level polarization scheme. In someembodiments, the first polarization state and the second polarizationstate correspond to data states in a two-bit multi-level polarizationscheme.

The bit line decoder 430 may be configured to selectively couple thevirtual ground sensing circuitry 500 to one or more of a number ofdifferent bit lines 102 (FIGS. 1 and 3). Thus, the bit line decoder 430may enable the control circuitry 400 to access many differentferroelectric memory cells 106 separately, together, in subgroups, etc.

FIG. 5A is a schematic diagram of the virtual ground sensing circuitry500. The virtual ground sensing circuitry 500 may include virtual groundcircuitry 530 configured to receive the bit line bias voltage potentialB/L BIAS from the biasing circuitry 410 (FIG. 4) and provide a virtualground held at a voltage potential B/L BIAS′ equal to the bit line biasvoltage potential B/L BIAS to the bit line 102 (e.g., through the bitline decoder 430 of FIG. 4). The virtual ground sensing circuitry 500may also include one or more sense circuits 540, 550 (sometimes referredto herein as “sense circuits” 540, 550) operably coupled to the virtualground circuitry 530. The sense circuits 540, 550 may be configured toreceive reference voltages V_(REF2), V_(REF1), respectively, from thebiasing circuitry 410 (FIG. 4). The sense circuits 540, 550 may beconfigured to use the reference voltages V_(REF2), V_(REF1),respectively, as references to detect sinked charge and emitted charge,respectively, to and from a ferroelectric memory cell 106 (FIGS. 1, 2A,3, 4) operably coupled to the bit line 102.

The bit line 102 may have a bit line capacitance C_(BL) (illustrated inFIG. 5A with broken lines to indicate that the bit line capacitanceC_(BL) is merely the capacitance of the bit line 102 itself, not anactual capacitor operably coupled to the bit line 102) associatedtherewith. The bit line capacitance C_(BL) may be a function of at leasta number and size of ferroelectric memory cells 106 operably coupled tothe bit line 102. For the sake of providing memory devices with highmemory capacities, relatively long bit lines 102 operably coupled to alarge number of ferroelectric memory cells 106 may be desirable.Accordingly, the bit line capacitance C_(BL) may be relatively large(e.g., on the order of a pF) in some cases.

The virtual ground circuitry 530 may include an operational amplifier532 operably coupled to a follower circuit 534. In some embodiments, theoperational amplifier 532 may include an operational transconductanceamplifier (OTA). A non-inverting input of the operational amplifier 532may be operably coupled to the biasing circuitry 410 to receive the bitline bias voltage potential B/L BIAS. An output of the operationalamplifier 532 may be operably coupled to an input of the followercircuit 534, and an output of the follower circuit 534 may be fed backto an inverting input of the operational amplifier 532.

The follower circuit 534 may include a pair of transistors Q1, Q2 in asource follower configuration. Transistor Q1 may include an n-type metaloxide semiconductor field effect transistor (n-MOS transistor), andtransistor Q2 may include a p-type MOSFET transistor (p-MOS transistor).The input of the follower circuit 534 may include gates of transistorsQ1 and Q2 operably coupled together. The output of the follower circuit534 may include sources of transistors Q1 and Q2 operably coupledtogether. The output of the follower circuit 534 may be operably coupledto the bit line 102 (e.g., through the bit line decoder 430 of FIG. 4).With the output of the follower circuit 534 coupled to the bit line 102,and fed back to the non-inverting input of the operational amplifier532, the bit line 102 may serve as a virtual ground at the voltagepotential B/L BIAS′ equal to the bit line bias voltage potential B/LBIAS. Accordingly, if charge is sinked to, or emitted from, aferroelectric memory cell 106 operably coupled to the bit line 102, thesinked or emitted charge may be drawn from, or discharged to, the bitline 102. The bit line 102 may have a bit line capacitance C_(BL)associated therewith.

Sense circuit 550 may be configured to sense charge emitted from aferroelectric memory cell 106 operably coupled to the bit line 102.Charge may be emitted from a ferroelectric memory cell 106 if theferroelectric memory cell 106 switches from a higher polarization stateto a lower polarization state (e.g., switching from P1 (+P_(R1)) to P2(−P_(R1)), from P2 (−P_(R1)) to P4 (−P_(R2)), from P3 (+P_(R2)) to P2(−P_(R1)), from P3 (+P_(R2)) to P4 (−P_(R2)), from P1 (+P_(R1)) to P4(−P_(R2)), etc.). Sense circuit 550 may include a comparator 552configured to output a data signal DATA1. An inverting input of thecomparator 552 may be operably coupled to the biasing circuitry 410(FIG. 4), and configured to receive a reference voltage V_(REF1). Anon-inverting input of the comparator 552 may be operably coupled to asense node SN1, which may, in turn, be operably coupled to a drain oftransistor Q2 of the follower circuit 534. The sense node SN1 may have asense node capacitance C_(SN1) associated therewith. In someembodiments, the sense node capacitance may be a parasitic capacitance(parasitic capacitance C_(SN1) is shown in broken lines to indicate thatit may be merely a parasitic capacitance, not a separate capacitor addedto the sensing circuitry 550). In some embodiments, the parasiticcapacitance C_(SN1) may include an actual capacitor operably coupled tothe sense node SN1. The sense node capacitance C_(SN1) may be relativelysmall compared to the bit line capacitance C_(BL).

The sense circuit 550 may also include an n-MOS transistor Q3 configuredto selectively operably couple the sense node SN1 to a low voltagepotential power supply V_(SS). A gate of transistor Q3 may be operablycoupled to a memory controller (not shown), and configured to receive acontrol signal CTRL1. Control signal CTRL1 may be held at a logic level1 to operably couple the sense node SN1 to V_(SS), except during a senseoperation to detect charge emitted from the ferroelectric memory cell106.

In some embodiments, a virtual ground sensing circuit includes anoperational amplifier including a non-inverting input, an invertinginput, and an amplifier output. The virtual ground sensing circuit alsoincludes a follower circuit including an n-MOS transistor and a p-MOStransistor. An input of the follower circuit includes a gate of then-MOS transistor operably coupled to a gate of the p-MOS transistor. Anoutput of the follower circuit includes a source of the n-MOS transistoroperably coupled to a source of the p-MOS transistor. The output of thefollower circuit is operably coupled to the inverting input of theoperational amplifier. The virtual ground sensing circuit furtherincludes a comparator configured to compare a sense node voltage at adrain of one of the n-MOS transistor and the p-MOS transistor to areference voltage potential. In some embodiments, the virtual groundsensing circuit includes another comparator configured to compareanother sense node voltage at a drain of the other of the n-MOStransistor and the p-MOS transistor to another reference voltagepotential. In some embodiments, the drain of the other of the n-MOStransistor and the p-MOS transistor is operably coupled to a powersupply voltage potential. In some embodiments, the drain of the one ofthe n-MOS transistor and the p-MOS transistor is operably coupled to apower supply voltage potential though a transistor configured to isolatethe drain of the one of the n-MOS transistor and the p-MOS transistorfrom the power supply voltage potential during a sense operation. Insome embodiments, the output of the follower circuit is operably coupledto a conductive line decoder configured to selectively operably couplethe output of the follower circuit to one of a plurality of conductivelines of a memory cell array. In some embodiments, the comparator isconfigured to compare a drain voltage potential of the p-MOS transistorto the reference voltage potential. In some embodiments, the virtualground sensing circuit includes a digital to analog converter (DAC)configured to provide the reference voltage potential to the comparator.A multi-bit digital signal swept from a low digital value to a highdigital value is applied to an input of the DAC during a senseoperation. In some embodiments, the virtual ground sensing circuitincludes a latch network configured to store a digital value of themulti-bit digital signal that corresponds to a data state of a selectedferroelectric memory cell operably coupled to the conductive line. Insome embodiments, the latch network is configured to be clocked by anoutput of the comparator.

In some embodiments, the virtual ground sensing circuit includes anoperational amplifier having a non-inverting input configured to receivethe second biasing signal, and a follower circuit including an n-MOStransistor and a p-MOS transistor. A source of the n-MOS transistor isoperably coupled to each of a source of the p-MOS transistor, aninverting input of the operational amplifier, and the second electrodeof the memory cell. A drain of the n-MOS transistor is operably coupledto a high voltage potential power supply. A drain of the p-MOStransistor is capacitively coupled to a low voltage potential powersupply. A gate of the n-MOS transistor and a gate of the p-MOStransistor are both operably coupled to an output of the operationalamplifier. The virtual ground sensing circuit further includes acomparator configured to compare a drain voltage potential of the drainof the p-MOS transistor to a reference voltage potential. In someembodiments, the drain of the p-MOS transistor is capacitively coupledto the low-voltage potential power supply through a parasiticcapacitance. In some embodiments, the control circuitry also includes abit line decoder configured to selectively operably couple the source ofthe n-MOS transistor to the second electrode of the memory cell.

FIG. 5B illustrates plots 560, 562, 564, 566 (566-1 and 566-2), and 568(568-1 and 568-2) of different voltage potentials V_(BL), CTRL1, V_(WL),V_(SN1), and DATA1, respectively, from the virtual ground sensingcircuit 500 plotted against time t during a sense operation of aselected ferroelectric memory cell 106 involving sense circuit 550 in asingle-level polarization scheme (i.e., only polarization levels P1(+P_(R1)) and P2 (−P_(R1)) are used). Referring to FIGS. 5A and 5Btogether, at time t=0, the biasing circuitry 410 (FIG. 4) may drive thenon-inverting input of the operational amplifier 532 to bias voltagepotential B/L BIAS. As a result, the virtual ground sensing circuitry500 may drive node BL at a bit line 102 operably coupled to the selectedferroelectric memory cell to B/L BIAS′ equal to B/L BIAS, as shown inthe plot 560 of FIG. 5B. Also, the node BL may serve as a virtual groundat the voltage potential B/L BIAS′.

At time t1, a memory controller (not shown) operably coupled to thevirtual ground sensing circuitry 500 may switch the control signal CTRL1at the gate of transistor Q3 from a logic level 1 to a logic level 0, asshown in the plot 562 of FIG. 5B. Accordingly, sense node SN1 may beisolated from the low voltage potential power supply V_(SS), except thatthe sense node SN1 may be capacitively coupled to the low voltagepotential power supply V_(SS) through the sense node capacitanceC_(SN1).

At time t2, the biasing circuitry 410 (FIG. 4) may drive a word line 104operably coupled to the selected ferroelectric memory cell 106 to biasvoltage potential W/L BIAS, as shown in plot 564 of FIG. 5B. Biasvoltage potentials W/L BIAS and B/L BIAS may be selected to applycritical voltage −V_(CR1) across the selected ferroelectric memory cell106. Bias voltage potential W/L BIAS may be applied at a sufficient timet2 following the assertion of B/L BIAS to allow transients on the bitline 102 to dissipate to steady state B/L BIAS.

If the selected ferroelectric memory cell 106 is in polarization stateP1 (+P_(R1)), the selected ferroelectric memory cell 106 may switch frompolarization state P1 (+P_(R1)) to polarization state P2 (−P_(R1)),emitting charge ΔQ from the selected ferroelectric memory cell 106 tothe bit line 102. The emitted charge ΔQ may slightly raise the voltagepotential V_(BL) on node BL (i.e., according to ΔV_(BL)=ΔQ/C_(BL)), andthe operational amplifier 532 may drive the input of the followercircuit 534 low, operably coupling bit line node BL to sense node SN1.The charge ΔQ from the selected ferroelectric memory cell 106 may bedischarged to the sense node SN1, and the voltage potential V_(SN1) ofthe sense node SN1 may rise (i.e., according to ΔV_(SN1)=ΔQ/C_(SN1)), asshown in plot 566-1.

As previously discussed, comparator 552 may be configured to compare thereference voltage V_(REF1) to the voltage V_(SN1) of the sense node SN1.The reference voltage V_(REF1) may be selected to be between the lowvoltage potential power supply V_(SS) and the voltage potential thatwould result on the sense node SN1 if the selected ferroelectric memorycell 106 emits charge ΔQ to the bit line 102 (i.e.,V_(SS)<V_(REF1)<(V_(SS)ΔQ/C_(SN1))). Accordingly, when the voltagepotential of the sense node V_(SN1) at time t3 surpasses the referencevoltage V_(REF1), as shown in the plot 566-1 of FIG. 5B, the comparator552 may switch from outputting a logic level 0 to a logic level 1 attime t3, as shown in the plot 568-1 of FIG. 5B.

If, however, the selected ferroelectric memory cell 106 is already inpolarization state P2 (−P_(R1)), the selected ferroelectric memory cell106 may not switch polarization states. Accordingly, the selectedferroelectric memory cell 106 may not emit charge ΔQ. Thus, charge ΔQmay not be conducted through transistor Q2 to the sense node SN1, andthe voltage potential V_(SN1) of sense node SN1 may remain unchanged attimes t2 and t3, as shown in plot 566-2 of FIG. 5B. Also, the voltagepotential V_(SN1) of the sense node SN1 may not surpass the referencevoltage V_(REF1). Accordingly, the comparator 552 may not switch fromoutputting the logic level 0 to the logic level 1 at time t3, as shownin the plot 568-2 of FIG. 5B.

It may, therefore, be determined whether the selected ferroelectricmemory cell was in polarization state P1 (+P_(R1)) or polarization stateP2 (−P_(R1)) by applying the critical voltage −V_(CR1) to the selectedferroelectric memory cell 106, and monitoring the output DATA1 of thecomparator 552. If DATA1 switches to logic 1 at time t3, the selectedferroelectric memory cell 106 was in polarization state P1. If it isdesired for the ferroelectric memory cell 106 to persist in polarizationstate P1 after the sense operation, the ferroelectric memory cell 106may then be biased to critical voltage V_(CR1) to switch thepolarization of the ferroelectric memory cell 106 back to polarizationstate P1 (because sensing the ferroelectric memory cell 106 switched thepolarization to state P2). If, however, DATA1 remains at logic 0, theselected ferroelectric memory cell 106 was in polarization state P2. Theferroelectric memory cell 106 may then be left in polarization state P2,if desired.

The polarization state of the selected ferroelectric memory cell 106 maybe determined using only the sense circuit 550, and not the sensecircuit 540 (or by using only the sense circuit 540, and not the sensecircuit 550, as will be discussed with reference to FIG. 5C). Thus, insome embodiments, the virtual ground sensing circuitry 500 may onlyinclude the virtual ground circuitry 530 and the sense circuit 550. Insuch embodiments, a drain of transistor Q1 of the follower circuit 534may be operably coupled to a high voltage potential power supply V_(DD)(shown in broken lines). In some embodiments, the virtual ground sensingcircuitry 500 may include both sense circuit 540 and sense circuit 550.

As previously discussed, the bit line capacitance C_(BL) may berelatively large. The sense node capacitance C_(SN1) on the may berelatively low, by comparison. Accordingly, a voltage change ΔV_(SN1) onthe sense node SN1 may be relatively large (i.e., ΔQ/C_(SN1)) comparedto a voltage change ΔV_(BL) (i.e., ΔQ/C_(BL)) on the bit line when thecharge ΔQ is emitted by the selected ferroelectric memory cell 106. As aresult, the virtual ground sensing circuitry 500 may be capable ofdetecting much smaller emitted charge ΔQ than conventional senseamplifiers, which typically compare the bit line voltage V_(BL) to areference voltage.

As the voltage change ΔV_(SN1) on the sense node SN1 is inverselyproportional to the sense node capacitance C_(SN1), the sensitivity ofthe virtual ground sensing circuitry 500 may increase as the sense nodecapacitance C_(SN1) decreases. Thus, it may be advantageous to providethe sense node with a capacitance C_(SN1) that is substantially lowerthan the bit line capacitance C_(BL). As previously discussed, in someembodiments, the sense node capacitance may include a parasiticcapacitance of the node SN1 to provide as small as possible a sense nodecapacitance C_(SN1), and therefore, to increase ΔV_(SN1).

Also, as previously discussed with reference to FIGS. 2A and 2B, thecharge ΔQ emitted from the selected ferroelectric memory cell 106 may beproportional to the area A of the selected ferroelectric memory cell 106(i.e., ΔQ=2(A)(P_(R1))). Accordingly, the virtual ground sensingcircuitry 500 may be capable of sensing ferroelectric memory cells 106that have much smaller area A than conventional sense amplifiers thatmonitor a bit line (or word line) voltage. Specifically, a ratio ofΔV_(SN1) to ΔV_(BL) may be C_(BL)/C_(SN1). It follows, then, that thevirtual ground sensing circuitry 500 may be capable of sensingferroelectric memory cells 106 having an area A that is smaller than anarea of ferroelectric memory cells sensed by conventional senseamplifiers by a factor of about C_(BL)/C_(SN1). If a small parasiticcapacitance C_(SN1) is achieved, it may be possible to senseferroelectric memory cells 106 with areas A that rival, or are evensmaller than, areas of known NAND flash memory cells.

It should be noted that the follower circuit 534 may have a “dead zone”where the input of the follower circuit does not trigger eithertransistor Q1 or transistor Q2 (i.e., the input of the follower circuitis less than a threshold voltage V_(TN) of transistor Q1, and greaterthan a threshold voltage of transistor V_(TP)). Accordingly, in orderfor the follower circuit 534 to discharge the charge emitted from/sinkedto the selected ferroelectric memory cell 106 to the sense node SN1, theopen-loop gain A_(OL) of the operational amplifier 532 should beselected according to A_(OL)≧(V_(TP)+V_(TN))/V_(BL). By way ofnon-limiting example, assuming that V_(TP)+V_(TN)=2 volts (V), and theopen loop gain A_(OL) is 10,000, the virtual ground sensing circuitry500 should be capable of detecting a change in bit line voltage ΔV_(BL)of 200 μV. Also, smaller changes in bit line voltage ΔV_(BL) may bedetected if the open loop gain A_(OL) is 10,000 is selected to behigher. It will be observed that the bit line capacitance C_(BL) and bitline resistance do not factor into these design equations. Although bitline resistance may factor into the access time of the selectedferroelectric memory cell 106, however, the access time forferroelectric memory cells may be on the order of a microsecond, whichis about a tenth of the current access time for known NAND Flash memorycells.

FIG. 5C illustrates plots 570, 572, 574, 576 (576-1 and 576-2), and 578(578-1 and 578-2) of different voltage potentials V_(BL), CTRL2, V_(WL),V_(SN2), and DATA2, respectively, from the virtual ground sensingcircuitry 500 plotted against time t during a sense operation of aselected ferroelectric memory cell 106 involving sense circuit 540 in asingle-level polarization scheme (i.e., only polarization levels P1(+P_(R1)) and P2 (−P_(R1)) are used). Referring to FIGS. 5A and 5Ctogether, sense circuit 540 may be configured to sense charge sinked toa ferroelectric memory cell 106 operably coupled to the bit line 102.Charge may be sinked to a ferroelectric memory cell 106 if theferroelectric memory cell 106 switches from a lower polarization stateto a higher polarization state (e.g., switching from P2 (−P_(R1)) to P1(+P_(R1))). Sense circuit 540 may include a comparator 542 configured tooutput a data signal DATA2. A non-inverting input of the comparator 542may be operably coupled to the biasing circuitry 410 (FIG. 4), andconfigured to receive a reference voltage V_(REF2). An inverting inputof the comparator 542 may be operably coupled to a sense node SN2, whichmay in turn be operably coupled to a drain of transistor Q1 of thefollower circuit 534. The sense node SN2 may have a sense nodecapacitance C_(SN2) associated therewith. In some embodiments, the sensenode capacitance may be a parasitic capacitance (parasitic capacitanceC_(SN2) is shown in broken lines to indicate that it may be merely aparasitic capacitance, not a separate capacitor added to the sensingcircuitry 550). The sense node capacitance C_(SN2) may be relativelysmall compared to the bit line capacitance C_(BL).

The sense circuit 540 may also include a p-MOS transistor Q4 configuredto selectively operably couple the sense node SN2 to a high voltagepotential power supply V_(DD). A gate of transistor Q3 may be operablycoupled to a memory controller (not shown), and configured to receive acontrol signal CTRL2. Control signal CTRL2 may be held at a logic level0 to operably couple the sense node SN2 to V_(DD), except during a senseoperation to detect charge sinked to the ferroelectric memory cell 106.

During a sense operation involving the sense circuit 540, at time t=0,the biasing circuitry 410 (FIG. 4) may drive the non-inverting input ofthe operational amplifier 532 to bias voltage potential B/L BIAS. As aresult, the virtual ground circuitry 500 may drive node BL at a bit line102 operably coupled to the selected ferroelectric memory cell 106 toB/L BIAS′ equal to B/L BIAS, as shown in the plot 570 of FIG. 5C. Also,the node BL may serve as a virtual ground at the voltage potential B/LBIAS′.

At time t1, a memory controller operably coupled to the virtual groundsensing circuitry 500 may switch the control signal CTRL2 at the gate oftransistor Q4 from a logic level 0 to a logic level 1, as shown in theplot 572 of FIG. 5C. Accordingly, sense node SN2 may be isolated fromthe high voltage potential power supply V_(DD), except that the sensingnode SN2 may be capacitively coupled to the high voltage potential powersupply V_(DD) through the sense node capacitance C_(SN2).

At time t2, the biasing circuitry 410 (FIG. 4) may drive a word line 104operably coupled to the selected ferroelectric memory cell 106 to biasvoltage potential W/L BIAS, as shown in plot 574 of FIG. 5C. Biasvoltage potentials W/L BIAS and B/L BIAS may be selected to applycritical voltage V_(CR1) across the selected ferroelectric memory cell106. Bias voltage potential W/L BIAS may be applied to the ferroelectricmemory cell 106 at a time t2 after the bit line is biased to B/L BIAS toallow any transients to dissipate to steady state.

If the selected ferroelectric memory cell 106 is in polarization stateP2 (−P_(R1)), the selected ferroelectric memory cell 106 may switch frompolarization state P2 (−P_(R1)) to polarization state P1 (+P_(R1)),sinking charge ΔQ to the selected ferroelectric memory cell 106 from thebit line 102. The sinked charge ΔQ may slightly decrease the voltagepotential V_(BL) on node BL (i.e., according to ΔV_(BL)=ΔQ/C_(BL)), andthe operational amplifier 532 may drive the input of the followercircuit 534 high, operably coupling bit line node BL to sensing nodeSN2. The charge ΔQ sinked to the selected ferroelectric memory cell 106may be drawn from the sense node SN2, and the voltage potential V_(SN2)of the sense node SN2 may decrease (i.e., according toΔV_(SN2)=ΔQ/C_(SN2)), as shown in plot 576-1.

As previously discussed, comparator 542 may be configured to compare thereference voltage V_(REF2) to the voltage V_(SN2) of the sense node SN2.The reference voltage V_(REF2) may be selected to be between the highvoltage potential power supply V_(DD) and the voltage potential thatwould result on the sense node SN2 if the selected ferroelectric memorycell 106 sinks charge ΔQ from the bit line 102 (i.e.,(V_(DD)−ΔQ/C_(SN2))<V_(REF2)<V_(DD)). Accordingly, when the voltagepotential of the sense node V_(SN2) at time t3 decreases below thereference voltage V_(REF2), as shown in the plot 576-1 of FIG. 5C, thecomparator 542 may switch from outputting a logic level 0 to a logiclevel 1 at time t3, as shown in the plot 578-1 of FIG. 5C.

If, however, the selected ferroelectric memory cell 106 is already inpolarization state P1 (+P_(R1)), the selected ferroelectric memory cell106 may not switch polarization states. Accordingly, the selectedferroelectric memory cell 106 may not sink charge ΔQ. Thus, charge ΔQmay not be conducted through transistor Q1 from the sense node SN2, andthe voltage potential V_(SN2) of sense node SN2 may remain unchanged attimes t2 and t3, as shown in plot 576-2 of FIG. 5C. In turn, thecomparator 542 may not switch from outputting the logic level 0 to thelogic level 1 at time t3, as shown in the plot 578-2 of FIG. 5C.

It may, therefore, be determined whether the selected ferroelectricmemory cell was in polarization state P1 (+P_(R1)) or polarization stateP2 (−P_(R1)) by applying the critical voltage V_(CR1) to the selectedferroelectric memory cell 106, and monitoring the output DATA2 of thecomparator 542. If DATA2 switches to logic 1 at time t3, the selectedferroelectric memory cell 106 was in polarization state P2 (−P_(R1)). Ifit is desired for the ferroelectric memory cell 106 to persist inpolarization state P2 (−P_(R1)) after the sense operation, theferroelectric memory cell 106 may then be biased to critical voltageV_(CR2) to switch the polarization of the ferroelectric memory cell 106back to polarization state P2 (because sensing the ferroelectric memorycell 106 switched the polarization to state P1). If, however, DATA2remains at logic 0, the selected ferroelectric memory cell 106 was inpolarization state P1. The ferroelectric memory cell 106 may then beleft in polarization state P1 (+P_(R1)), if desired.

Similar to the sense circuit 550 discussed above with reference to FIGS.5A and 5B, the polarization state of the selected ferroelectric memorycell 106 may be determined using only the sense circuit 540 (without thesense circuit 550). Thus, in some embodiments, the virtual groundsensing circuitry 500 may only include the virtual ground circuitry 530and the sense circuit 540. In such embodiments, a drain of transistor Q2of the follower circuit 534 may be operably coupled to a low voltagepotential power supply V_(SS) (shown in broken lines). As previouslydiscussed, the virtual ground sensing circuitry 500 may include bothsense circuit 540 and sense circuit 550.

FIGS. 6A and 6B illustrate a sense circuit 550′ and plots of relatedvoltage potentials according to embodiments of the disclosure. FIG. 6Ais a schematic view of a sense circuit 550′ that may replace the sensecircuit 550 of the virtual ground sensing circuitry 500 of FIG. 5A. FIG.6B illustrates plots 630, 640 of voltage potentials V_(SN1), DATA 1,respectively of the sense circuit 550′ of FIG. 6A.

Referring to FIGS. 5A and 6A together, the sense circuit 550′ may besimilar to the sense circuit 550 except the sense circuit 550′ may beconfigured to detect emitted charge ΔQ resulting from polarizationchanges in a multi-level polarization scheme (e.g., polarization statesP1, P2, P3, P4, etc., may be used). Although the present discussionrefers to a two-bit sensing scheme using polarization levels P1, P2, P3,and P4 (corresponding to polarization levels +P_(R1), −P_(R1), +P_(R2),and −P_(R2), respectively), the disclosure is not so limiting. Forexample, a three-bit scheme including polarization levels P1, P2, P3,P4, P5, and P6 may be used, and so on to four-bit schemes, etc., so longas the ferroelectric memory cells 106 exhibit multi-level ferroelectricstates due to different critical voltages applied thereto.

The sense circuit 550′ may include a comparator 552′, a digital toanalog converter (DAC) 610, and a latch network 620. A non-invertinginput of the comparator 552′ may be operably coupled to sense node SN1,similar to the comparator 552 of the sense circuit 550 of FIG. 5.Accordingly, the non-inverting input of the comparator 552′ may beoperably coupled to a drain of transistor Q2 of the follower circuit 534of FIG. 5A.

An input of the DAC 610 may be configured to receive a digital signalBITS1 (e.g., from the biasing circuitry 410 of FIG. 4, from a memorycontroller, etc.). An output of the DAC 610 may be operably coupled toan inverting input of the comparator 552′.

The latch network 620 may include latches, each of the latchesconfigured to store a digital bit. If the digital signal BITS1 includestwo bits, then the latch network 620 may include at least two latches.If the digital signal BITS2 includes three bits, the latch network 620may include at least three bits, etc. An input of the latch network 620may be configured to receive the digital signal BITS1. Accordingly, theinput of the latch network 620 may be operably coupled to the input ofthe DAC 610. The latch network 620 may also include a clock input CLK.The clock input CLK may be operably coupled to an output of thecomparator 552′. Accordingly, the latch network 620 may be configured tostore digital bits applied by BITS1 to the input of the latch network620 when the output of the comparator 552 toggles from high to low.

The digital signal BITS1 may include a bus of digital bit signals (e.g.,a two-bit signal, a three-bit signal, a four-bit signal, etc.). During asense operation, the digital signal BITS1 may be swept from low to highdigital values. Accordingly, an output of the DAC 610 may provide avoltage signal V_(REF1)′ that steps up from a low voltage potentiallevel to a high voltage potential level.

Referring now to FIGS. 5A, 5B, 6A, and 6B together, the virtual groundsensing circuitry 500 including the sense circuit 550′ may perform asense operation. Plots 560, 562, and 564 of FIG. 5B corresponding toV_(BL), CTRL1, and V_(WL) are the same for the multi-level polarizationscheme. B/L BIAS and W/L BIAS, however, may be selected to apply acritical voltage potential corresponding to a polarization level with ahighest magnitude and a negative orientation (e.g., −V_(CR2)) to aselected ferroelectric memory cell 106. By way of non-limiting example,in a two-bit polarization scheme, the P4 (−P_(R2)) polarization levelhas the highest magnitude and negative polarity (out of P1 (+P_(R1)), P2−P_(R1)), P3 (+P_(R2)), and P4 (−P_(R2))). Accordingly, B/L BIAS and W/LBIAS may be selected to apply the critical voltage potential −V_(CR2) tothe selected ferroelectric memory cell 106.

When the critical voltage potential −V_(CR2) is applied to the selectedferroelectric memory cell 106, the selected ferroelectric memory cell106 may switch to polarization state P4 (−P_(R2)), unless the selectedferroelectric memory cell 106 was already in polarization state P4.Accordingly, charge ΔQ may be emitted from the selected ferroelectricmemory cell 106 when the critical voltage potential −V_(CR2) is appliedto the selected ferroelectric memory cell 106 if there is a change inpolarization state. The magnitude of the charge ΔQ emitted from theselected ferroelectric memory cell 106 may be different depending onwhether the ferroelectric memory cell 106 switches from polarizationstate P1 to P4 (ΔQ_(P1)), from polarization state P2 to P4 (ΔQ_(P2)), orfrom polarization state P3 to P4 (ΔQ_(P3)).

As previously discussed with reference to FIG. 5B, a voltage changeΔV_(SN1) at the sense node SN1 of the sense circuit 550′ may be greaterby a factor of C_(BL)/C_(SN1) than a voltage change ΔV_(BL) on the bitline 102 as a result of any charge ΔQ emitted by the selectedferroelectric memory cell 106. The change in voltage ΔV_(SN1) at thesense node SN1 may be ΔQ_(P1)/C_(SN1) for a switch from polarizationstate P1 (+P_(R1)) to P4 (−P_(R2)), ΔQ_(P2)/C_(SN1) for a switch frompolarization state P2 (−P_(R1)) to P4 (−P_(R2)), ΔQ_(P3)/C_(SN1) for aswitch from polarization state P3 (+P_(R2)) to P4 (−P_(R2)), and nochange if the polarization state stays at P4 (−P_(R2)). Plot 630 of FIG.6B illustrates the change in V_(SN1) that occurs at time t2 (when thecritical voltage potential −V_(CR2) is applied to the selectedferroelectric memory cell 106) for each of the different polarizationstates P1, P2, P3, P4 the selected memory cell 106 may have been in whenthe critical voltage −V_(CR2) was applied. As illustrated in the plot630, switching from polarization state P3 to P4 results in the largestchange in V_(SN1). Switching from P1 to P4 results in less change inV_(SN1). Switching from P2 to P4 results in still less change inV_(SN1). Of course, staying in P4 results in no change in V_(SN1).

After the critical voltage −V_(CR2) has been applied to the selectedferroelectric memory cell 106, the digital signal BITS1 may be sweptfrom low to high. As a result, V_(REF1)′ may exhibit a stepping pattern,as shown in the plot 630. Each step in the stepping pattern of V_(REF1)′may be associated with a different digital value of BITS1 driving theDAC 610. By way of non-limiting example, a digital 00 may be associatedwith a first step, a digital 01 may be associated with a second step, adigital 10 may be associated with a third step, and a digital 11 may beassociated with a fourth step. As long as the step size of V_(REF1)′ isapproximately equal to the distance between the different possiblevoltage levels of V_(SN1) (resulting from the different voltage changesin V_(SN1) when different charges are emitted responsive to changes inpolarization states of the selected ferroelectric memory cell 106), thedifferent digital signals (e.g., 00, 01, 10, 11) of BITS1 may also beassociated with the different polarization states P1, P2, P3, P4 (e.g.,P4 may be associated with 00, P2 may be associated with 01, P1 may beassociated with 10, and P3 may be associated with 11).

As illustrated in plot 630, V_(REF1)′ may intersect V_(SN1) at timet_(P4) if the selected ferroelectric memory cell 106 was in polarizationstate P4. Also, V_(REF1)′ may intersect V_(SN1) at time t_(P2) if theselected ferroelectric memory cell 106 was in polarization state P2.Furthermore, V_(REF1)′ may intersect V_(SN1) at time t_(P1) if theselected ferroelectric memory cell 106 was in polarization state P1. Inaddition, V_(REF1)′ may intersect V_(SN1) at time t_(P3) if the selectedferroelectric memory cell 106 was in polarization state P3. As a result,the output DATA1 of the comparator 552′, which is configured to compareV_(REF1)′ to V_(SN1), toggles from a logic 1 to a logic 0 at a differenttime depending on the polarization state of the selected ferroelectricmemory cell 106, as illustrated in plot 640.

When the output DATA1 of the comparator 552′ toggles to a logic 0, thelatch network 620 may store the digital signal BITS1 asserted at theinput of the latch network 620, which is the same as the digital signalBITS1 driving the DAC 610 (and V_(REF1)′, by extension). Accordingly, inthe example of FIG. 6B, if the polarization state of the selectedferroelectric memory cell 106 was P4, the latch network 620 will store a00. Also, if the polarization state was P2, the latch network 620 willstore a 01. If the polarization state was P1, the latch network 620 willstore 10. If the polarization state was P3, the latch network 620 willstore 11. Thus, by applying the critical voltage −V_(CR2), and sweepingthe digital signal BITS1, and reading data stored by the latch network620, it may be determined what data state the selected ferroelectricmemory cell 106 was in.

Of course, this sense operation switches the selected ferroelectricmemory cell 106 to polarization state P4 regardless of the polarizationstate the selected ferroelectric memory cell 106 was in before.Accordingly, if it is desired to preserve the data that was stored inthe selected ferroelectric memory cell 106, the polarization stateassociated with the digital signal 00, 01, 10, 11 may be re-applied tothe selected ferroelectric memory cell 106 by applying the correspondingcritical voltage V_(CR1), −V_(CR1), V_(CR2), −V_(CR2).

In some embodiments, rather than sweeping the digital signal BITS1 fromlow to high, the digital signal BITS1 may instead be swept from high tolow.

In some embodiments, rather than using steps of the reference voltageV_(REF1) that are approximately equal to the differences in V_(SN1)depending on the different polarization states of the selectedferroelectric memory cell 106, the steps may be smaller than thedifferences in V_(SN1). In such embodiments, the digital signal BITS1may include more bits than are stored in the ferroelectric memory cell106, and only some of the possible combinations of the bits in BITS1 maycorrespond to the different polarization states of the ferroelectricmemory cell 106.

In some embodiments, the comparator 552′ may instead compare an analogreference voltage sweeping linearly from low to high or from high to lowto V_(SN1), and the analog reference voltage may be applied to an analogto digital converter, which then provides a digital signal correspondingto the analog voltage to the latch network 620. The output of thecomparator 552′ may clock the latch network 620 to store a digital valuecorresponding to the value of the analog voltage that toggled thecomparator. In this configuration, digital values may be assigned to thedifferent polarization states similarly to the example discussed withrespect to FIGS. 6A and 6B.

In some embodiments, a “parallel” sensing scheme for sensing V_(SN1) maybe used. For example, multiple comparators may each be configured tocompare V_(SN1) to a different one of a plurality of different referencevoltages, each selected to correspond to a different data state 00, 01,10, 11. By analyzing the outputs of the comparators (e.g., observingwhich comparators toggle), the polarization state (and therefore thecorresponding data state) may be determined. Other sense circuitconfigurations are also contemplated within the scope of the disclosure.

FIGS. 7A and 7B illustrate an embodiment of a sense circuit 540′according to embodiments of the disclosure. FIG. 7A is a schematic viewof the sense circuit 540′ that may replace the sense circuit 540 of thevirtual ground sensing circuitry 500 of FIG. 5A. FIG. 7B illustratesplots 770, 780 of voltage potentials of the sense circuit 540′ of FIG.7A.

Referring to FIGS. 5A and 7A together, the sense circuit 540′ may becomplementary to the sense circuit 550′ discussed above with referenceto FIG. 6A (i.e., configured to detect charges sinked to the selectedferroelectric memory cell instead of charges emitted from the selectedferroelectric memory cell 106, as is the case with the sense circuit550′). The sense circuit 540′ may be similar to the sense circuit 540except the sense circuit 540′ may be configured to detect sinked chargeΔQ resulting from polarization changes in a multi-level polarizationscheme (e.g., polarization states P1, P2, P3, P4, etc., may be used).Although the present discussion refers to a two-bit sensing scheme usingpolarization states P1, P2, P3, and P4 (corresponding to polarizationlevels +P_(R1), −P_(R1), +P_(R2), and −P_(R2), respectively), thedisclosure is not so limiting. For example, a three-bit scheme includingpolarization levels P1, P2, P3, P4, P5, and P6 could be used, and so onto four-bit schemes, etc., so long as the ferroelectric memory cells 106exhibit multi-level ferroelectric states due to different criticalvoltages applied thereto.

The sense circuit 540′ may include a comparator 542′, a digital toanalog converter (DAC) 750, and a latch network 760. An inverting inputof the comparator 542′ may be operably coupled to sense node SN2,similar to the comparator 542 of the sense circuit 540 of FIG. 5A.Accordingly, the inverting input of the comparator 542′ may be operablycoupled to a drain of transistor Q1 of the follower circuit 534 of FIG.5A.

An input of the DAC 750 may be configured to receive a digital signalBITS2 (e.g., from the biasing circuitry 410 of FIG. 4, from a memorycontroller, etc.). An output of the DAC 750 may be operably coupled toan inverting input of the comparator 542′.

The latch network 760 may include latches, each of the latchesconfigured to store a digital bit. If the digital signal BITS2 includestwo bits, then the latch network 760 may include at least two latches.If the digital signal BITS2 includes three bits, the latch network 760may include at least three bits, etc. An input of the latch network 760may be configured to receive the digital signal BITS2. Accordingly, theinput of the latch network 620 of FIG. 6A may be operably coupled to theinput of the DAC 750. The latch network 760 may also include a clockinput CLK. The clock input CLK may be operably coupled to an output ofthe comparator 542′. Accordingly, the latch network 760 may beconfigured to store digital bits applied by BITS2 to the input of thelatch network 760 when the output of the comparator 542′ toggles fromhigh to low.

The digital signal BITS2 may include a bus of digital bit signals (e.g.,a two-bit signal, a three-bit signal, a four-bit signal, etc.). During asense operation, the digital signal BITS2 may be swept from high to lowdigital values. Accordingly, an output of the DAC 750 may provide avoltage signal V_(REF2)′ that steps down from a high voltage potentiallevel to a low voltage potential level.

Referring now to FIGS. 5A, 5C, 7A, and 7C together, the virtual groundsensing circuitry 500 including the sense circuit 540′ may perform asense operation. Plots 570, 572, and 574 of FIG. 5C corresponding toV_(BL), CTRL2, and V_(WL) are the same for the multi-level polarizationscheme. B/L BIAS and W/L BIAS, however, may be selected to apply acritical voltage potential corresponding to a polarization level with ahighest magnitude and a positive orientation (e.g., V_(CR2)) to aselected ferroelectric memory cell 106. By way of non-limiting example,in a two-bit polarization scheme, the P3 (+P_(R2)) polarization levelhas the highest magnitude and positive polarity (out of P1 (+P_(R1)), P2−P_(R1)), P3 (+P_(R2)), and P4 (−P_(R2))). Accordingly, B/L BIAS and W/LBias may be selected to apply the critical voltage potential V_(CR2)(FIGS. 2A and 2B) to the selected ferroelectric memory cell 106.

When the critical voltage potential V_(CR2) is applied to the selectedferroelectric memory cell 106, the selected ferroelectric memory cell106 may switch to polarization state P3 (+P_(R2)), unless the selectedferroelectric memory cell 106 was already in polarization state P3.Accordingly, charge ΔQ may be sinked to the selected ferroelectricmemory cell 106 when the critical voltage potential V_(CR2) is appliedto the selected ferroelectric memory cell 106 if there is a change inpolarization state. The magnitude of the charge ΔQ sinked to theselected ferroelectric memory cell 106 may be different depending onwhether the ferroelectric memory cell 106 switches from polarizationstate P1 to P3 (ΔQ_(P1)), from polarization state P2 to P3 (ΔQ_(P2)), orfrom polarization state P4 to P3 (ΔQ_(P4)).

As previously discussed with reference to FIG. 5C, a voltage changeΔV_(SN2) at the sense node SN2 of the sense circuit 540′ may change by afactor of C_(BL)/C_(SN2) as compared to a voltage change ΔV_(BL) on thebit line 102 a result of any charge ΔQ sinked to the selectedferroelectric memory cell 106. The change in voltage ΔV_(SN2) at thesense node SN2 may be ΔQ_(P1)/C_(SN2) for a switch from polarizationstate P1 (+P_(R1)) to P3 (+P_(R2)), ΔQ_(P2)/C_(SN2) for a switch frompolarization state P2 (−P_(R1)) to P3 (+P_(R2)), ΔQ_(P4)/C_(SN2) for aswitch from polarization state P4 (−P_(R2)) to P3 (+P_(R2)), and nochange if the polarization state stays at P3 (+P_(R2)). Plot 770 of FIG.7B illustrates the change in V_(SN2) that occurs at time t2 (when thecritical voltage potential V_(CR2) is applied to the selectedferroelectric memory cell 106) for each of the different polarizationstates P1, P2, P3, P4 the selected memory cell 106 may have been in whenthe critical voltage V_(CR2) was applied. As illustrated in the plot770, switching from polarization state P4 (−P_(R2)) to P3 (+P_(R2))results in the largest change in V_(SN2). Switching from P2 (−P_(R1)) toP3 (+P_(R2)) results in less change in V_(SN2). Switching from P1(+P_(R1)) to P3 (+P_(R2)) results in still less change in V_(SN2). Ofcourse, staying in P3 (+P_(R2)) results in no change in V_(SN2).

After the critical voltage V_(CR2) has been applied to the selectedferroelectric memory cell 106, the digital signal BITS2 may be sweptfrom high to low. As a result, V_(REF2)′ may exhibit a stepping pattern,as shown in the plot 770. Each step in the stepping pattern of V_(REF2)′may be associated with a different digital value of BITS2 driving theDAC 610 of FIG. 6A. By way of non-limiting example, a digital 11 may beassociated with a first step, a digital 10 may be associated with asecond step, a digital 01 may be associated with a third step, and adigital 00 may be associated with a fourth step. As long as the stepsize of V_(REF2)′ is approximately equal to the distance between thedifferent possible voltage levels of V_(SN2) (resulting from thedifferent voltage changes in V_(SN2) when different charges are sinkedresponsive to changes in polarization states of the selectedferroelectric memory cell 106), the different digital signals (e.g., 00,01, 10, 11) of BITS2 may also be associated with the differentpolarization states P1, P2, P3, P4 (e.g., P3 may be associated with 11,P1 may be associated with 10, P2 may be associated with 01, and P3 maybe associated with 00).

As illustrated in plot 770, V_(REF2)′ may intersect V_(SN2) at timet_(P3) if the selected ferroelectric memory cell 106 was in polarizationstate P3 (+P_(R2)). Also, V_(REF2)′ may intersect V_(SN2) at time t_(P1)if the selected ferroelectric memory cell 106 was in polarization stateP1 (+P_(R1)). Furthermore, V_(REF2)′ may intersect V_(SN2) at timet_(P2) if the selected ferroelectric memory cell 106 was in polarizationstate P2 (−P_(R1)). In addition, V_(REF2)′ may intersect V_(SN2) at timet_(P4) if the selected ferroelectric memory cell 106 was in polarizationstate P4 (+P_(R1)). As a result, the output DATA2 of the comparator542′, which is configured to compare V_(REF2)′ to V_(SN2), toggles froma logic 1 to a logic 0 at different times depending on the polarizationstate of the selected ferroelectric memory cell 106, as illustrated inplot 780.

When the output DATA2 of the comparator 542′ toggles to a logic 0, thelatch network 760 may store the digital signal BITS2 asserted at theinput of the latch network 760, which is the same as the digital signalBITS2 driving the DAC 750 (and V_(REF2)′, by extension). Accordingly, inthe example of FIG. 7B, if the polarization state of the selectedferroelectric memory cell 106 was P3 (+P_(R2)), the latch network 760will store a digital 11. Also, if the polarization state was P1(+P_(R1)), the latch network 760 will store a digital 10. If thepolarization state was P2 (−P_(R1)), the latch network 760 will store adigital 01. If the polarization state was P4 (−P_(R2)), the latchnetwork 760 will store a digital 00. Thus, by applying the criticalvoltage V_(CR2), sweeping the digital signal BITS2, and reading datastored by the latch network 760, it may be determined what polarizationstate P1, P2, P3, P4 the selected ferroelectric memory cell 106 was in.

Of course, this sense operation switches the selected ferroelectricmemory cell 106 to polarization state P3 (+P_(R2)) regardless of thepolarization state the selected ferroelectric memory cell 106 was inbefore. Accordingly, if it is desired to preserve the data that wasstored in the selected ferroelectric memory cell 106, the sensedpolarization state associated with the digital signal 00, 01, 10, 11 maybe re-applied to the selected ferroelectric memory cell 106 by applyingthe corresponding critical voltage V_(CR1), −V_(CR1), V_(CR2), −V_(CR2).

FIG. 8 is a simplified flowchart 800 of a method of performing a senseoperation of a selected ferroelectric memory cell 106. Referring toFIGS. 4, 5A, 6A, 7A, and 8 together, at operation 810, the method mayinclude providing a virtual ground at a first bias voltage potential(e.g., B/L BIAS′) to a conductive line (e.g., bit line 102, word line104) operably coupled to a selected ferroelectric memory cell 106 at apolarization state (e.g., one of P1, P2, P3, P4, etc., corresponding torespective polarization levels +P_(R1), −P_(R1), +P_(R2), −P_(R2),etc.).

At operation 820, the method may include applying a second bias voltagepotential (e.g., W/L BIAS) to another conductive line (e.g., the otherof bit line 102 and word line 104) operably coupled to the selectedferroelectric memory cell 106. The first bias voltage potential and thesecond bias voltage potential may be selected to apply a criticalvoltage potential V_(CR1), −V_(CR1), V_(CR2), −V_(CR2), etc., to theselected ferroelectric memory cell 106.

At operation 830, the method may include operably coupling theconductive line (e.g., 102, 104) to a sense node SN1, SN2 of a sensecircuit 540, 540′, 550, 550′ having a sense node capacitance C_(SN1),C_(SN2) if the selected ferroelectric memory cell 106 switches toanother polarization state P1, P2, P3, P4 (corresponding respectively topolarization levels +P_(R1), −P_(R1), +P_(R2), −P_(R2)).

At operation 840, the method may include comparing a voltage potentialV_(SN1), V_(SN2) at the sense node SN1, SN2 to a reference voltagepotential V_(REF1), V_(REF1)′, V_(REF2), V_(REF2)′ to determine thepolarization state of the selected ferroelectric memory cell 106.

At operation 850, the method may include reprogramming the selectedferroelectric memory cell 106 to the polarization state if the selectedferroelectric memory cell 106 switches to the another polarizationstate.

In some embodiments, a method includes providing a virtual ground at afirst bias voltage potential to a conductive line operably coupled to aselected ferroelectric memory cell in a first polarization state. Themethod also includes applying a bias voltage potential to anotherconductive line operably coupled to the selected ferroelectric memorycell. The first bias voltage potential and the second bias voltagepotential are selected to apply a critical voltage to the selectedferroelectric memory cell. The method also includes operably couplingthe conductive line to a sense node of a sense circuit having a sensenode capacitance if the ferroelectric memory cell switches to a secondpolarization state. The method further includes comparing a sense nodevoltage potential at the sense node to a reference voltage potential todetermine the first polarization state. In some embodiments, the methodincludes storing data corresponding to the determined first polarizationstate in a latch, and resetting the selected ferroelectric memory cellto the first polarization state after the sense operation is performed.In some embodiments, the method includes sweeping the reference voltagepotential from the low voltage potential to the high voltage potentialduring the sense operation. In some embodiments, sweeping the referencevoltage potential from the low voltage potential to the high voltagepotential includes sweeping the reference voltage potential in astepping pattern from the low voltage potential to a high voltagepotential. In some embodiments, sweeping the reference voltage potentialfrom the low voltage potential to the high voltage potential includessweeping the reference voltage potential linearly from the low voltagepotential to the high voltage potential.

In some embodiments, a method of operating an electrical system includesapplying, with biasing circuitry, a critical voltage potential to aselected ferroelectric memory cell in an array of ferroelectric memorycells through a pair of conductive lines operably coupled to theselected ferroelectric memory cell. The critical voltage potential isselected to cause the selected ferroelectric memory cell to switch froma first polarization state to a second polarization state. The methodalso includes discharging, with virtual ground sensing circuitryoperably coupled to one of the pair of conductive lines, charge that isat least one of sinked to and emitted from the selected ferroelectricmemory cell to a sense node if the selected ferroelectric memory cellswitches from the first polarization state to the second polarizationstate. The sense node has a sense node capacitance that is less than acapacitance of the one of the pair of conductive lines. In someembodiments, the method may further comprise determining that theselected ferroelectric memory cell was in the first polarization stateresponsive to detecting the charge that is discharged to the sense node.In some embodiments, the method may further include determining that theselected ferroelectric memory cell was in the second polarization stateresponsive to detecting zero change in charge at the sense node. In someembodiments, the method may further include determining that theselected ferroelectric memory cell was in a third polarization statedifferent from the first polarization state and the second polarizationstate responsive to detecting a different charge that is discharged tothe sense node.

FIG. 9 is a simplified block diagram of a computing device 900 includinga memory device 910 that includes the control circuitry 400 of FIG. 4.The computing device 900 may include a processing circuit 920 operablycoupled to the memory device 910, one or more input devices 940 and oneor more output devices 950. The memory device 910 may include an array100 of memory cells (e.g., the ferroelectric memory cells 106 of FIG. 1)operably coupled to the control circuitry 400. The control circuitry 400may include the virtual ground sensing circuitry (VGSC) 500, aspreviously discussed herein. The processing circuit 920 may beconfigured to execute computer-readable instructions stored in the array100.

The input devices 940 may include a keyboard, a mouse, a track pad, amicrophone, a touch-screen, other input devices, and combinationsthereof. The output devices 950 may include an electronic display (e.g.,a touch screen), an acoustic transducer, light emitting diodes, otheroutput devices, and combinations thereof. The input device 940 and theoutput devices may be configured to enable a user of the computingdevice 900 to interact with the computing device.

In some embodiments, a computing device includes a memory device. Thememory device includes an array of ferroelectric memory cells, andcontrol circuitry including a virtual ground sensing circuit. Thevirtual ground sensing circuit is configured to operably couple to theferroelectric memory cells of the array of ferroelectric memory cells.The virtual ground sensing circuit is configured to provide a virtualground to a conductive line operably coupled to a selected ferroelectricmemory cell, and selectively discharge the conductive line to a sensenode of a sense circuit responsive to the selected ferroelectric memorycell switching from a first polarization state to a second polarizationstate. The sense node has a sense node capacitance less than acapacitance of the conductive line. The computing device also includes aprocessing circuit operably coupled to the memory device. The processingcircuit is configured to execute computer-readable instructions storedin the array of the memory device. In some embodiments, the computingdevice includes at least one input device and at least one output deviceoperably coupled to the processing circuit. The at least one inputdevice and the at least one output device are configured to enable auser of the computing device to interact with the computing device.

While certain illustrative embodiments have been described in connectionwith the figures, those of ordinary skill in the art will recognize andappreciate that embodiments encompassed by the disclosure are notlimited to those embodiments explicitly shown and described herein.Rather, many additions, deletions, and modifications to the embodimentsdescribed herein may be made without departing from the scope ofembodiments encompassed by the disclosure, such as those hereinafterclaimed, including legal equivalents. In addition, features from onedisclosed embodiment may be combined with features of another disclosedembodiment while still being encompassed by the disclosure.

1. A virtual ground sensing circuit, comprising: an operationalamplifier comprising a non-inverting input, an inverting input, and anamplifier output; a follower circuit including an n-MOS transistor and ap-MOS transistor, an input of the follower circuit including a gate ofthe n-MOS transistor operably coupled to a gate of the p-MOS transistor,and an output of the follower circuit including a source of the n-MOStransistor operably coupled to a source of the p-MOS transistor, theoutput of the follower circuit operably coupled to the inverting inputof the operational amplifier; and a comparator configured to compare asense node voltage at a drain of one of the n-MOS transistor and thep-MOS transistor to a reference voltage potential.
 2. The virtual groundsensing circuit of claim 1, further comprising another comparatorconfigured to compare another sense node voltage at a drain of the otherof the n-MOS transistor and the p-MOS transistor to another referencevoltage potential.
 3. The virtual ground sensing circuit of claim 1,wherein a drain of the other of the n-MOS transistor and the p-MOStransistor is operably coupled to a power supply voltage potential. 4.The virtual ground sensing circuit of claim 1, wherein the drain of theone of the n-MOS transistor and the p-MOS transistor is operably coupledto a power supply voltage potential through a transistor configured toisolate the drain of the one of the n-MOS transistor and the p-MOStransistor from the power supply voltage potential during a senseoperation.
 5. The virtual ground sensing circuit of claim 1, wherein theoutput of the follower circuit is operably coupled to a conductive linedecoder configured to selectively operably couple the output of thefollower circuit to one of a plurality of conductive lines of a memorycell array.
 6. The virtual ground sensing circuit of claim 1, whereinthe comparator is configured to compare a drain voltage potential of thep-MOS transistor to the reference voltage potential.
 7. The virtualground sensing circuit of claim 1, wherein the sense node has a sensenode capacitance.
 8. The virtual ground sensing circuit of claim 7,wherein the sense node capacitance includes a parasitic capacitance. 9.The virtual ground sensing circuit of claim 1, further comprising one ormore other comparators configured to compare the sense node voltage toone or more other reference voltage potentials.
 10. The virtual groundsensing circuit of claim 1, further comprising a digital to analogconverter (DAC) configured to provide the reference voltage potential tothe comparator, wherein a multi-bit digital signal swept from a lowdigital value to a high digital value is applied to an input of the DACduring a sense operation.
 11. The virtual ground sensing circuit ofclaim 10, further comprising a latch network configured to store adigital value of the multi-bit digital signal that corresponds to a datastate of a selected memory cell operably coupled to the conductive line.12. The virtual ground sensing circuit of claim 11, wherein the latchnetwork is configured to be clocked by an output of the comparator. 13.The virtual ground sensing circuit of claim 1, wherein the operationalamplifier is an operational transconductance amplifier.
 14. Anelectrical system, comprising: at least one memory cell; and controlcircuitry operably coupled to the at least one memory cell, the controlcircuitry comprising: an operational amplifier comprising anon-inverting input, an inverting input, and an amplifier output; afollower circuit comprising a first transistor and a second transistor,an input of the follower circuit comprising a gate of the firsttransistor operably coupled to a gate of the second transistor, and anoutput of the follower circuit comprising a source of the firsttransistor operably coupled to a source of the second transistor, theoutput of the follower circuit operably coupled to the inverting inputof the operational amplifier; and a comparator configured to compare asense node voltage at a drain of one of the first transistor and thesecond transistor to a reference voltage potential.
 15. The electricalsystem of claim 14, wherein the control circuitry further comprises afeedback circuit operably coupling the output of the comparator circuitto the inverting input of the operational amplifier and to a conductiveline.
 16. The electrical system of claim 15, further comprises theconductive line operably coupled to the at least one memory cell. 17.The electrical circuit of claim 14, further comprising a digital toanalog converter (DAC) configured to receive a digital signal, an outputof the DAC operably coupled to an inverting input of the comparator. 18.A method of performing a sense operation for a selected memory cell, themethod comprising: providing a virtual ground at a first bias voltagepotential to a conductive line operably coupled to the selected memorycell; applying a second bias voltage potential to another conductiveline operably coupled to the selected memory cell, wherein the firstbias voltage potential and the second bias voltage potential areselected to apply a critical voltage to the selected memory cell;operably coupling the conductive line to a follower circuit comprisingat least one transistor; operably coupling a drain of the at least onetransistor to a sense node of a sense circuit having a sense nodecapacitance; and comparing a sense node voltage potential at the sensenode to a reference voltage potential.
 19. The method of claim 18,further comprising capacitively coupling the sense node to a low voltagepower supply.
 20. The method of claim 19, wherein capacitively couplingthe sense node to the low voltage power supply comprises: receiving acontrol signal from a memory controller; and operably coupling the sensenode to the low voltage power supply responsive to the received controlsignal.